A method for amplifying an echo signal suitable for detection of a vehicle's surroundings is discussed in publication DE 10 2008 054 789 A1, in which a gain factor is set as a function of a transit time of the echo signal. A variable amplifier is provided for this purpose, the gain curve optionally also having a sudden course in multiple steps. A complex operational amplifier circuit in which resistances are activated and deactivated for a sudden change in gain is shown as an exemplary embodiment. This also describes how the curve of the sudden change in gain must additionally be calibrated by a central signal processor.
The same publication describes how in systems, which sense the surroundings and carry out pulse-wise measurements, the evaluation complexity may be reduced with the aid of a transit time-dependent gain V(τ) since the signal strength becomes lower with an increase in echo transit time. This describes a continuous change in the input gain which may take place since the received signal is much less falsified by a continuous change in comparison with a sudden change.
In systems of an optimal configuration, it should be possible to vary the gain V(τ) in the range of
      1    ≤                  V        ⁡                  (          τ          )                            V        0              <          V      max        ,where Vmax>100 from the standpoint of the system.
FIGS. 1a and 1b show two possible curves of a desired transit time-dependent signal gain V(τ) as examples.
It is also believed to be understood from the related art how a transit time dependent gain may be implemented with little technical complexity by suitable wiring of an A/D converter, which may be in a recursive loop containing the signal reception, a certain part of the signal evaluation and optionally a suitable arrangement of generating a control signal for transit time-dependent gain regulation of an A/D converter in particular.
The recursive loop is not necessary for implementation of the transit time-dependent gain. If it is used, however, the transit time-dependent gain may be adapted to the prevailing signal situation, whereby the required dynamic range of the A/D converter may be reduced.
However, the technical implementation of amplifiers with a continuously adjustable gain is very difficult in particular because these amplifiers should also have a suitable dynamic range for the gain of incoming signals in addition to the adjustable gain range.
However, the implementation of N amplifiers having a fixed gain is implementable with very little effort. A very high gain
      V          fixed_total      ⁢      _N        =            ∏              j        =        1            N        ⁢          V              fixed        ⁢                                  ⁢        j            may be implemented by a series circuit with such amplifiers and the signal is present partially amplified at the transitions between the amplifiers in each case:
      V          fixed_total      ⁢                          ⁢      k        =            ∏              j        =        1            k        ⁢                  V                  fixed          ⁢                                          ⁢          j                    .      
An equivalent situation applies for A/D converters. The range in which the reference voltage of A/D converters may be varied without any negative feedback effects on the quality of the A/D conversion is typically only in the range of 1 to 4.
There are therefore neither inexpensive A/D converters nor inexpensive variably adjustable amplifiers with which the required variable gain range
  1  ≤            V      ⁡              (        τ        )                    V      0        <      V    max  is implementable at approximately 50 kHz.